关于PDF中FieldMDP字典含P键的合规性及处理方式问询

Great question—this is exactly the kind of real-world PDF quirk that comes up when dealing with digital signatures, since not all tools strictly follow the letter of the spec. Let’s break this down clearly:

First, Confirm the Spec Definition #

Per the PDF 32000-1:2008 specification:

• Page 733: The P entry is explicitly part of DocMDP transform dictionaries, where it defines the document-wide modification permission level (e.g., no changes allowed, only form filling, etc.).
• Page 736: The FieldMDP transform dictionary has no defined P entry. Its valid entries focus on field-specific permissions (like V for field modification rules) and action bindings.

How to Treat the Unexpected P Entry #

1. Follow the Spec’s Core Rule for Undefined Dictionary Entries #

The PDF spec explicitly states in section 7.3.7 ("Dictionary Objects") that conforming readers must ignore any entries not explicitly defined for a given dictionary type. This is a foundational rule to ensure compatibility across tools—non-standard entries shouldn’t break document processing.

So, from a strict compliance standpoint, you should ignore the P entry in the FieldMDP dictionary. It has no official, standardized meaning in this context.

2. Why Might It Be There? #

In practice, this is almost certainly one of two things:

• A bug or oversight in the third-party tool that generated the signature—whoever built the tool may have confused DocMDP and FieldMDP structures.
• A private, non-standard extension from a specific vendor. Even if that vendor treats it as meaningful, there’s no guarantee other PDF readers will recognize or respect it. You can’t rely on it having consistent behavior across different tools.

3. Impact on Signature Validity #

Ignoring the P entry won’t affect the validity of the digital signature itself. The signature hash covers the entire signed dictionary, but validation tools only check compliance against standard entries when evaluating permission rules. The extra P key is just unrecognized metadata in this case.

Final Takeaway #

Stick to the specification: the P entry in a FieldMDP dictionary has no defined meaning and should be ignored by conforming PDF processors. Any perceived functionality from this key would be non-standard and tool-specific, not part of official PDF rules.

内容的提问来源于stack exchange，提问作者NedStarkOfWinterfell